The present invention relates to FM radar systems, and in particular, to FM radar systems for use in collision avoidance systems.
A radar system which is mounted on a vehicle such as an automobile and used in conjunction with an alarm system to detect and warn of potential frontal and rear end collisions, must have high resolution and a minimum range of several decimeters. For this reason, it has been recognized that for use in collision warning systems, frequency modulated-continuous wave (FM-CW) radar systems are preferred over conventional pulsed radar systems. Further, as the longest required range of detection is roughly several hundred meters, it is preferred to utilize a beam having a frequency of about 60 GHz. The reasons for this are that beams having a frequency in the 60 GHz range are easily attenuated upon propagation, and that the use of such beams avoids interference with microwave transmission systems already in existence.
An exemplary prior art FM Radar System is disclosed in U.S. Pat. No. 5,181,037 entitled "FM Radar System." FIG. 4 is a block diagram of that system. As shown, a voltage of a triangular wave form is produced by sweep circuit 121, which operates under the control of timing control circuit 120. The triangular wave form voltage is supplied to a variable frequency oscillator 110 to produce a microwave FM signal. The FM signal is divided into two parts, each of which is supplied to an up-converter 112 or 113 respectively. The up-converters 112 and 113 convert each part of the divided signal to a microwave FM signal having a frequency in the range of about 20 GHz. Each of the converted FM signals is supplied to one of four frequency triple multipliers 114a-114d or one of four frequency triple multipliers 115a-115d successively through either of switches 122 or 123 which operate in sync with a timing signal supplied from the timing control circuit 120. FM mm-wave signals having a frequency of about 60 GHz, which are output successively from frequency triple multipliers 114a-114d, are supplied to antennas 118a-118d through circulators 116a-116d. The signals are then radiated from the antennas 118a-118d successively.
In addition, FM mm-wave signals which are output successively from frequency triple multipliers 115a-115d, are supplied successively to a first input terminal of mixers 117a-117d. FM mm-wave signals, which are reflected from a target and received by antennas 118a-188d, are supplied successively to a second input terminal of mixers 117a-117d through circulators 116a-116d. Beat signals, which are output successively from mixers 117a-117d, are supplied to detecting circuit 119 through switching circuit 124. The operating frequencies of local oscillators of the up-converters 112 and 113 are set to different values to shift the frequency of the beat signals output from mixers 117a-117d to a frequency range high enough to reduce interference from 1/f noise.
Since wave guide type circulators 116 are used in the FM radar systems of the prior art, an example of which is illustrated in FIG. 4, the size of prior art systems becomes large and the systems become expensive. Size of FM radar systems can be reduced by using microstrip type circulators instead of wave guide type circulators, however, the use of microstrip type circulators substantially increases the cost of an FM radar system.
In addition, it will be noted by those skilled in the art that sufficient isolation to suppress interference between transmitter and receiver cannot be realized using microstrip type circulators. Finally, microstrip type circulators have a large insertion loss.